The subject application is related to subject matter disclosed in Japanese Patent Application No. H11-276626 filed on Sep. 29, 1999 in Japan to which the subject application claims priority under Paris convention and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a planar filter constituted by disposing a filter member opposite to a tuning member, particularly to a technique of using a superconductor as a filter material for use in a communication apparatus and the like.
2. Related Background Art
In a communication apparatus for performing information communication by radio or cable, a filter for extracting only a desired frequency band is an important constituting component. To realize the effective use of a frequency, and energy savings, a filter superior in attenuation property and small in insertion loss is demanded.
To satisfy such a demand, a resonance element high in a Q value is necessary as a filter constituting element. As one technique of realizing the resonance element with a high Q value, there has been proposed a technique of using a superconductor as a conductor constituting the resonance element, and using a material with a very low loss such as sapphire or MgO in a substrate. In this technique, a Q value of 10000 or more can be obtained, and a resonance property becomes improved. On the other hand, there is a problem that the resonance property has to be adjusted with a high degree of accuracy when designing and making the filter.
That is, by a slight dispersion of permittivity of the substrate or a slight processing error of the conductor during processing, the resonance property largely changes, and a desired filter property cannot be obtained. Moreover, even when the desired filter property is obtained, there is also a problem that a deviation is generated in the filter property of variation with time or ambient temperature change.
On the other hand, a technique of utilizing the aforementioned high Q value and directly filtering a high frequency signal of a GHz band is proposed in order to omit a frequency converter and realize cost reduction. Also in this case, needless to say, the resonance property of the resonance element has to be highly precisely adjusted, but if an arbitrary frequency can be selected with one filter by positively changing a resonance frequency, a filter constitution can be simplified, and the cost reduction can be achieved.
Additionally, as a technique of eliminating the aforementioned filter property deviation, for example, there is a technique of disposing, on the resonance element, a dielectric whose permittivity changes depending upon voltage, and disposing a voltage applying electrode in the vicinity of the dielectric.
In this technique, by variably controlling an electrode arrangement place and the applied voltage, the permittivity can locally and independently be changed. As a result, this enables individual and independent adjustments of: (1) the resonance frequency of the resonance element, (2) coupling between the resonance elements, and (3) coupling between the resonance element and input/output portion, which are usually necessary for tuning a pass frequency band of the filter. Specifically, the pass frequency band be can variably controlled, and a skirt property and ripple can be adjusted so that desired properties are obtained. Here, the skirt property indicates rise and fall properties of both the sides of the pass frequency band, and the ripple indicates a property recess degree in the pass frequency band. Usually, it is preferable that the skirt property be steep and the ripple be small.
However, in a conventional technique, the dielectric for changing the permittivity, and the electrode for applying the voltage are essential constituting elements, losses by the dielectric and electrode lowers the Q value of the resonance element down to several hundreds or less, and it is difficult to obtain the resonance element and filter superior in attenuation property and small in insertion loss.
Another technique is to dispose, on a resonator of a micro-strip structure, a magnetic (YIG) plate whose permeability changes in accordance with an applied magnetic field, and uniformly apply the magnetic field to the plate from the outside in order to change the resonance frequency.
In this technique, as compared with the aforementioned dielectric control system, no electrode is necessary, a YIG loss is smaller than that of the dielectric, and the Q value of the resonance element can therefore be improved by a factor of ten. However, when this technique is applied to tune the filter property, only the uniform magnetic field can be applied to the respective resonance elements and between the resonance elements or to the input/output portion, the individual and independent adjustments of the aforementioned adjustments (1) to (3), necessary for tuning the filter pass frequency band, are therefore impossible, and there is a problem that changing of the pass frequency band deteriorates the skirt property and ripple.
The present invention has been developed in consideration of the aforementioned problems, and an object thereof is to provide a planar filter which can variably control a pass frequency band with a high precision, and which is superior in skirt property and in ripple property.
Another object of the present invention is to provide a planar filter which can individually and independently adjust a resonance frequency of a resonance element as a filter constituting component, coupling between the resonance elements, and coupling between the resonance element and an input/output portion.
Further object of the present invention is to provide a planar filter which can tune a pass frequency band at a high speed and in a broad range with a simple constitution without sacrificing a low loss property of a superconductor.
To achieve the aforementioned objects, there is provided a planar filter comprising:
a filter member in which a plurality of resonance elements of superconductor thin films and input/output portions disposed on both the sides of the resonance elements are formed via gaps on a dielectric substrate; and
a tuning member which is formed of a magnetic material disposed opposite to the filter member via a predetermined gap and to which a direct-current magnetic field is applied.
The tuning member includes a permittivity adjusting section which can adjust an effective permittivity of at least one of a gap periphery between the resonance elements and a gap periphery between the input/output portion and the resonance element.
According to the present invention, the filter member is disposed opposite to the tuning member, and the tuning member can adjust the effective permittivity of at least one of the gap periphery between the resonance elements in the filter member, and the gap periphery between the input/output portion and the resonance element. Because of this, when changing the filter pass frequency band, the skirt property can be improved, and the ripple can be eliminated.
Moreover, there is provided a planar filter comprising:
a filter member in which a plurality of resonance elements of superconductor thin films and input/output portions disposed on both the sides of the resonance elements are formed via gaps on a dielectric substrate; and
a tuning member disposed opposite to the filter member via a predetermined gap.
The tuning member comprises:
a first magnetic material disposed opposite to a gap between the input/output portion and the resonance element;
a second magnetic material disposed opposite to each of the resonance elements;
a third magnetic material disposed opposite to a gap between the resonance elements; and
magnetic field generation means for adjusting the permeability of the first to third magnetic materials.
According to the present invention, by disposing the tuning member including the first to third magnetic materials opposite to the filter member, and adjusting the permeability of the first to third magnetic materials, the resonance frequency, the coupling between the resonance elements, and the coupling between the resonance element and the input/output portion can variably be controlled, and the skirt property, ripple, and other filter properties can be improved.